This invention relates to evaporative emission control systems for internal combustion engines, and more particularly, to an actuator having a permanent magnet for increasing armature force and displacement.
Many motor vehicles utilize actuators such as solenoids to operate several types of devices. This includes devices such as a fuel cell valve or an exhaust gas recirculation (EGR) valve used in an EGR system. In such systems, the EGR valve is controlled by a circuit in accordance with various engine operating conditions to regulate the amount of engine exhaust gas that is recirculated back into the engine for combustion. This serves to limit the combustion temperature and hence reduce the formation of oxides of nitrogen.
Solenoids typically utilize an electromagnet coil to generate a magnetic force which causes an armature to move along an axis. The armature may be part of a mechanism for operating a valve, such as an EGR valve. Referring to FIG. 1, an enlarged view of a first magnetic flux density 10 located near portions of a lower stator 12, upper stator 46, gap 36, first armature 14 and coil 16 of a conventional solenoid is shown. It is noted that the configuration shown is substantially symmetrical about an axis of the solenoid and that only one side of the axis is shown for purposes of clarity. The lower stator 12 has a frusto-conical shape having a predetermined geometry and is separated from the upper stator 46 by a gap 36. The shape of the lower stator 12 along with the size of the gap 36 and other parameters are selected so as to optimize a flux path that forms a part of a magnetic circuit. This provides a desired solenoid characteristic in that the armature force is substantially constant with respect to armature displacement. Referring to FIG. 1, the magnetic flux density vectors are oriented in a substantially clockwise configuration in a lower portion 18 of the first armature 14 adjacent to the lower stator 12. In addition, the magnetic flux density vectors are relatively dispersed along an edge 20 of the lower portion 18. In this configuration, the magnetic flux density in an upper section 22 of the lower stator 12 ranges from approximately 1597 to 2195 kiloGauss (kGauss).
It is desirable that solenoids used in motor vehicles provide increased armature force and increased travel so as to improve controllability and increase flow. However, this would require larger solenoids and the amount of space available in current vehicle engine compartments is limited.
A solenoid which includes a housing having a coil for generating a first magnetic field. An armature is slidably mounted in the housing. The solenoid further includes a permanent magnet having a second magnetic field, wherein the magnet is located adjacent the armature. In addition, a stator is affixed in the housing for cooperation with the armature and the magnet to form a third magnetic field.
The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, may be best understood by reference to the following description taken in conjunction with accompanying drawings.